Said schematic operation can basically be traced back to traditional luminaires, which use a ballast composed of a core of small ferromagnetic plates in a pack, on which a metal conductor coil is wound, powered by alternating current mainly supplied by public electric networks. Since the ballast load is highly inductive, it is usually necessary to add, in the lamp power circuit, a power factor correction capacitor.
However, as described in document EP 1063870, the use is also known of traditional transformers, with primary and secondary windings, intended to increase the saturation current of the secondary winding, when the discharge lamp starting module is miniaturised, as is the case, for example in motor vehicle headlights.
Although such technology is still widespread, both for the many systems still operating, but also for new systems in particular for lighting streets and outdoor structures, various alternative solutions have been adopted, aiming to overcome the defects and limits of ferromagnetic ballasts. Such limits basically consist of some noisiness during operation caused by lamellar pack vibrations, the poor tolerance of voltage oscillations on the electric networks at various times of day, accentuated thermal dispersion, with consequent reduction of component lifetime, and above all low overall energy efficiency obtainable with such ballasts.
Therefore, alternative electronic devices are currently spreading, such as the self-oscillating ballast, which needs neither the ignitor (or starter), since it can use a trigger voltage produced inside it, nor the power factor correction capacitor, since the power factor is high enough. Moreover, the overall efficiency of the luminaire is considerably higher than that obtained using ferromagnetic ballasts.
Other devices use an electronic control circuit for varying the lamp power supply voltage and frequency, making said parameters optimal for the various phases which come one after another, such as filament heating, initial discharge, stable operation or variations in the brightness of the lamp.
An example is described in document U.S. Pat. No. 4,275,337, in which an electronic ignitor is also able to very quickly re-start a discharge lamp which has been switched off.
In general, the above-mentioned various types of electronic devices for powering discharge lamps, although working very well, are not good at withstanding prolonged exposure to weather, particularly at the high temperatures which occur in systems for lighting streets or outdoor areas, with energy efficiency and technical performance which rapidly deteriorate over time, even necessitating their early substitution.
Another disadvantage which must be considered is the high cost of such electronic devices. Said particular features mean that even today traditional ferromagnetic ballasts are still widely used.